Work over the past two years has demonstrated that a significant degree of remineralization of both natural and artificial lesions occurs after exposure to a calcifying fluid in vitro. The aim of the first part of the study is to investigate the degree of remineralization produced using short exposure times which would be acceptable clinically. The degree of remineralization produced over one hour exposure increments will be investigated as will single and multiple six-minute exposures. With respect to lesion progression, work will be carried out to compare the effect of remineralization of the enamel surface superficial to a lesion, relative to a two-step regimen in which the lesion is remineralized in depth followed by high surface remineralization. Parallel studies will also be carried out in which small natural lesions will be exposed to the artificial caries medium to cause lesion progression. The effect of the calcifying fluid on sound enamel surfaces will also be studied as will its use on acid-etched enamel. Structural changes produced, and the susceptibility of such remineralized surfaces to caries in vitro, will be investigated. All of the work to date relating to the production of caries-like lesions and their remineralization has been carried out on "clean" enamel surfaces. This was essential to eliminate variables of exogenous origin. Now the data must be made more realistic in terms of the clinical situation. Work in the second part of this study will therefore be carried out to investigate the effect pellicle has on lesion initiation, progress and remineralization. In each series of experiments, the enamel surface on one half of a tooth will be maintained "clean" whilst the surface on the adjacent tooth half will be covered with pellicle formed in vitro. It is essential to find out just what effect pellicle has on the various lesion parameters and how these are modified when pellicles of different ages are employed.